It is known to provide coolable tube walls for metallurgical vessels or furnaces that allow the lining to be kept relatively cool, that is as compared to the high refining or other treatment temperatures, and hence rigid. Such systems normally have a succession of laterally spaced straight tube sections joined at their ends by reverse bends or full 180.degree. elbows, and joined laterally by rigid webs that form of the tube wall a rigid self-supporting structure. The bends have a radius of curvature which determines the spacing of the tubes, and the width of the webs. A cooler is connected to such an assembly to pass a coolant such as water or steam through it.
The fabrication of such a tube wall is an extremely complex procedure, involving meticulous interfitting of the various parts and then welding them together along seams that run the full length of each tube section, between it and the adjacent web plate and annularly around each tube end and the respective end of the reverse-bend coupling. Thus each tube has two annular welds connecting it to the respective reverse bends, and two full-longitudinal welds connecting it to the respective webs, with yet another semicircular weld extending as a continuation of the respective full-longitudinal weld between each web end and the inside of the respective reverse bend. Obviously all such welds must be carefully inspected, particularly where they cross, a location where the second weld is liable to weaken the first.
Even the most carefully made such structure is subjected to great stresses from thermal expansion and contraction, due principally to the difference in temperature between the interconnecting webs and the tubes themselves. The webs are, obviously, much warmer than the tubes, so that the welds between them and the tubes are under enormous strain. Since these welds are very long, it is possible for the stresses between the relatively hot tubes and relatively cool webs to be therefore effective in turn along the entire tube length, placing the system under great strain.
In addition such systems are problematic because these long weld seams along the walls of the tubes slightly weaken these tubes in this region. Thus it is necessary to make the tubes of stock which is sufficiently thick to withstand the considerable pressures it will be subject to in use, even though such thickness is only needed at the weakened weld regions. In other words, if it were not for these webs it would be possible to make the tube sections of substantially lighter stock, rated purely for the pressure they will have to withstand, without allowing for the weakening caused by welding the webs to them.